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Automatic Transfer Switch (ATS). The Key to High Reliability Source Switching. Presentation Overview. ATS – The Key to High Reliability Source Switching

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automatic transfer switch ats

Automatic Transfer Switch (ATS)

The Key to High Reliability Source Switching


Presentation Overview

  • ATS – The Key to High Reliability Source Switching
  • Various technologies are used to switch critical loads from a primary to a backup power source. When nothing goes wrong in the facility (overloads, short circuits, etc.) most technologies perform adequately. When problems occur, certain ATS technologies will keep the power on…others will not. Knowing the differences and correctly applying ATS in your facility is key to providing high reliability source switching.
  • This presentation will include:
    • A definition of ATS
    • An overview of typical ATS applications, constructions, and types
    • A review of the IEC codes and testing procedures for ATS
    • A comparison of the performance of each technology under IEC testing –
    • under normal and abnormal circumstances
    • Application guidelines for selectively coordinated, high reliability, fault-
    • tolerant source switching.
    • Overall recommendations for a Power Quality installations.

Section 1 – ATS Basics

  • What is an ATS ?
  • What are typical applications of ATS?
  • Common constructions of ATS
  • Common types of ATS

What is an Automatic Transfer Switch (ATS) ?

  • An ATS is a device designed to Automatically switch (transfer) between (2) sources of power to improve the reliability of the electrical supply to a connected load.

Backup Source

(typically a Generator Supply

(Normally De-Energized)


Primary Source

(typically a Utility Supply or Utility Bus)

(Normally Energized)

typical applications of ats
Typical Applications of ATS
  • Utility and Engine Generator
    • Single unit standby applications are very common
  • Utility and Utility
    • Dual utility applications are becoming more common
  • Generator and Generator
    • Prime power / standby application












Types of ATS construction

GE Power Quality

GE components

Market Trend

typical ats constructions knife blade
Typical ATS ConstructionsKnife Blade
  • Typically Non-Load breaking
  • Usually Manually Operated (and slow)
  • Sometimes motor operated (still slow)
  • Low withstand ratings – no arc chutes or vents
  • Low endurance ratings = limited life
typical ats constructions twin contactor
Typical ATS ConstructionsTwin Contactor

IEC 60947-4-1

  • Switching device having 1 position of rest (IEC def’n)
  • Returns to position of rest @ loss power (un-latched type) or add’l latching arrangement to hold position on loss of power
  • Built using 1 of up to 12 types of contactors under IEC 60947-4-1 Code, from basic contactor to high-capacity motor starters.
  • Often very basic controller
  • Mechanical/safety interlock often not available or optional
  • Variety of operators, from motor to electromagnets
  • Not a True double throw device - possible to close into both Sources
typical ats constructions twin mccb molded case circuit breaker
Typical ATS ConstructionsTwin MCCB (Molded Case Circuit Breaker)

IEC 60947-2

  • 2 Position device, typically operated from add-on motors
  • Built using 1 of up to 2 types of breaker under IEC 60947-2 Code, from basic instantaneous-trip type to short-time rated devices.
  • Mechanical/safety interlock is an add-on, sometimes optional
  • Motor operators usually slow, often unreliable for high duty.
  • Wide range of manf-assigned withstand ratings (low, med, or high)
  • Not a True double throw device - possible to close into both Sources
typical ats constructions power contactor type ge zenith std
Typical ATS ConstructionsPower Contactor Type (GE Zenith Std.)

UL 1008

IEC 60947-6-1

  • Std. Double Throw mechanism - Inherent mechanical interlock against closing to both sources
  • No add-on interlocks required
  • Fewer moving parts
  • High Withstand Current ratings to allow fault clearing coordination
  • Integrated Arc quenching components
  • Solenoid Operated (very fast)
power contactor ats composition
Power Contactor ATS - Composition
  • Arc quenching grids & enclosed arc chutes
  • Movable contact assembly has 2 pieces, arcing contact and main current carrying contact.
  • Arcing contacts make first, break last [See Video].
  • Silver alloy contacts
    • Resists welding
    • Enhanced withstand ratings
  • “Over center” switching principal to achieve a mechanically locked position in either Source 1 or 2
  • High speed solenoid actuated drive assures contact transfer in 30-50 msec

(Click to see video)

typical ats constructions static
Typical ATS ConstructionsStatic






  • Two live sources required. Solid-state switching >> no moving parts
  • Extremely FAST operation, transfer in less than ¼ cycle.
  • SCR technology
  • Typically used on critical loads where 2 reliable and independent power supplies available, or switching 2 x UPS Outputs.
  • Both sources must be in sync
  • Often cost prohibitive for Standby applications



(No Center Position)

Delayed Transition

(With Center OFF Position)

NOTE: “OFF” Position indicatesNeither Source connected

Closed Transition

(Make-Before-Break Operation)

NOTE: “P” Position indicates both sources paralleled for 5 cycles

Standard Transfer Switch: Types







Bypass Isolation ATS

  • Essentially (2) ATS in Parallel, (1) Automatic Switch (ATS) & (1) Manual Switch (MTS).
  • ATS is withdrawable, similar to withdrawable CB’s
  • (2) redundant paths from each source to load.
  • ATS & MTS are mechanically and electrically interlocked
      • To prevents accidental closure of both sources
  • Bypassing power from Source to Load permits testing or maintenance of ATS without interruption to load.
  • Available in Open, Delayed, and Closed Transition
  • If power fails while bypassed, Genset is auto started to permit fast transfer using MTS.

Section 2 - Codes & Standards for equipment applied as ATS

  • What are the Internationally-

recognized codes?

  • What is the Performance difference

of devices tested to these codes?


Internationally-recognized ATS Codes and Standards

Devices arranged for Source Switching, but not IEC rated as ATS

Devices arranged for Source Switching, IEC rated as ATS

Contactors & Motor Starters



Contactors & Motor Starters



Power Contactors

IEC 60947-4-1

UL 508

IEC 60947-2

UL 489

IEC 60947-6-1

UL 1008

Devices may be arranged to function as an ATS, but not be rated as ATS by a Recognized body


IEC Performance Testing of ATS - Overview

GE ATS Ratings

Twin CB’s

Twin Contactors

CB: Breaker type

PC/CC: Contactor types.



Circuit Breaker


IEC Code

Transfer Switching Equipment 60947-6-1

(AC-31A, 31B)

(AC-33A, 33B)

(IEC ‘A’)

(IEC ‘AC-1’)

Util. Cat.

31A: High Operations @ Rated Current33A: High Operations @ 2 x Rated Current

31B: Low Operations @ Rated Current

33B: Low Operations @ 2 x Rated Current

Operational Performance

High Operations @

Rated current

Low Operations @ majority No Current

Overload or Low current short circuit Performance

31A: High Operations @ 1.5 Rated Current33A: High Operations @ 6x Rated Current

31B: Low Operations @ 1.5 Rated Current

33B: Low Operations @ 6x Rated Current

Low Operations @

6x Rated Current, High pf, for up to 630A Rating. (N/A >630A)

Low Operations @

1.5x Rated Current, High pf

CC/PC Type:

Break SC: No

Make SC: Yes

Make & Hold: Yes

Withstand SC: Yes

CB Type:

Break SC: Yes

Make SC: Yes

Make & Hold: No

Withstand SC: No

High/Short Circuit Performance

Break SC: Yes

Make SC: Yes

Make & Hold: No

Withstand SC: No

No Requirements

short circuit testing of pc type ats
Short Circuit Testing of PC-Type ATS
  • Contactors and Circuit Breakers have Fault current ratings, expressed in kAIC
  • PC-type ATS are tested to:
    • Withstand fault currents (remain closed), for a set period of time, permitting downstream devices to trip and isolate the fault.
    • Close into faults, to permit the Backup supply fault current necessary to clear the fault.
    • For this reason, PC-type ATS have Withstand and Close Ratings (WCR) vs. Traditional kAIC Fault rating (as in a CB).

FaultTesting (click to see video)


Performance of ATS Technologies - Summary

CB: Breaker type

PC/CC: Contactor types.

Twin CB’s

Twin Contactors

“Real life” performance of ATS in your facility

Building Event

Low Operation switching of Rated Load


Not OK


33A, 33B

High Duty Cycle/Switching


Not OK

31A, 31B

OK (up to 630A)

Switching Motors w/ 6-10X inrush @ start

33A, 33B

Not OK


31A, 31B

Short on load side of ATS

Need upstream SCPD to trip

Inst. Trip.

Selectivity w/ downstream SCPD ???

Same as CB

Designed to withstand & permit downstream SCPD

to clear

Designed to close into and withstand to permit downstream SCPD to clear

Switching into a short circuit on load side of ATS.

Same as above

Big problem


PQ Application Recommendations for ATS

  • Specify ATS certified As an ATS to IEC 60947-6-1 or UL 1008 Un-certified ATS have not been tested for the purposes of source switching.
  • Specify an ATS with adequate duty cycle rating

“A” or “B” IEC Utilization Catagories, based on application

  • Specify an ATS with adequate overload capability for switching inductive loads

“31” or “33” IEC Utilization Catagories, based on application

  • Specify PC Type of ATS for critical load application …

Specify PC-type where fault withstand, vs, tripping under faults, is required for proper system Selective Coordination for faults below ATS.